In recent years, gaseous fuels such as hydrogen and compressed natural gas (CNG: Compressed Natural Gas) other than liquid fuels of gasoline, diesel fuel and the like have been widely used as fuels for automobiles. In particular, CNG draws attention as a clean energy source based on advantages such as (i) calorific value per unit CO2 emission is large and (ii) there is no emission of sulfur oxide, because sulfur component is hardly included. In the case of applying CNG to an automobile engine, similarly to gasoline and diesel fuel in the past, it is injected from a fuel injector into an intake port or cylinder of the engine.
Also, with respect to liquid fuels of gasoline, diesel fuel and the like, evaporation is promoted by being injected from a fuel injector at a high pressure and by supplying the liquid fuel as fine droplets. Here, in many cases of the gasoline engine, the injector is located at a portion of an intake manifold in the vicinity of the intake valve and the fuel injection pressure becomes around 3 atm.
Recently, a system in which gasoline is injected directly into the cylinder, that is, an engine of a system referred to as GDI (Gasoline Direct Injection) has been taken into consideration. In this system, a stratified mixture of gasoline and air is formed in the cylinder and according to this system, it is possible to move an automobile with less fuel, but the pressure in the cylinder is high, so that it is necessary to supply atomized gasoline into the cylinder with more pressure than that, for example, with injection pressure of around 100 atm. The fuel injection duration at one stroke in this case is around several msec to several dozen msec and at that time, the duration required for the opening and closing of an electromagnetic valve that is opened and closed by a solenoid becomes an extremely short duration of around several hundred nsec to several msec.
Fuel flow rate per unit time (m3/sec) in the case of supplying liquid fuel into the cylinder by opening and closing the electromagnetic valve is referred to as the injection rate, and accurate measurement of the injection rate is required. Generally, when an injection valve is in an opening state, the injection rate increases as the opening duration increases and is converged with a constant value (saturated state). Then, when the injection valve is in a closing state, there is a tendency of decreasing the injection rate along with the elapse of time and it returns to the original state. As an instrument (instantaneous flow rate meter) for measuring the injection rate (synonymous with “instantaneous flow rate”) from an liquid fuel injector, a system referred to as Bosch type (Bosch type injection rate measuring device) is widely used (for example, see Non-patent Document 1).
The Bosch type injection rate measuring device employs a method in which the fuel from the liquid fuel injector is injected into a pipe having a constant cross-sectional area and the fuel flow rate is obtained from the increase in pressure in the pipe. This uses such a fundamental principle that in the case of injecting the fuel into a thin tube from the fuel injector the injection rate (m3/sec) is obtained by multiplying the thin tube cross-sectional area (m2) and flow velocity (m/sec). More specifically, it is a system in which when the fuel injected from the injector flows in a thin tube, then, the pressure increase occurs as a result thereof, and the instantaneous flow rate of the fuel is obtained by measuring the pressure increase.
However, although it is possible to use the Bosch type injection rate measuring device for the measurement of the liquid fuel, it was not possible to use it for a gaseous fuel such as hydrogen and compressed natural gas (CNG). More specifically, in the case of applying the Bosch type device to a gaseous fuel injector, there is a problem that the measurement of flow becomes impossible because of compressibility of the gas, that is, property in which density varies together with pressure change, and increase in pipe friction or the like caused by the increase of Reynolds number. Up to now, an instantaneous injection rate of a gaseous fuel has hardly been measured.
Here, the Reynolds number Re is referred to as a dimensionless value obtained by dividing a value ρ*U*D(kg/(m·sec)), which is obtained by multiplying the fluid density ρ (kg/m3) and the fluid velocity U (m/sec) and the diameter D(m) of the thin tube, by the viscosity coefficient μ (Pa·sec=kg/(m·sec)). The ρ*U*D of the fluid relates to inertial force showing the force of flow and the viscosity coefficient μ relates to viscous force showing viscous behavior. When the Reynolds number Re is small, the viscous behavior prevails, so that laminar flow without turbulence occurs and the measurement of the injection rate is possible, but when the Reynolds number is large, the gas flow becomes turbulent (turbulent flow occurs) and the measurement of the injection rate becomes impossible due to the fact that it is difficult for the gas to flow by the pipe friction. Actually, it is known that a turbulent flow state may occur when the Reynolds number Re is approximately 2500 or more and a laminar flow occurs when it is approximately 2500 or less.
Generally, in a gaseous fuel injection, the Reynolds number Re becomes as much as several hundred thousands and the flow in the tube is turbulent, so that pressure difference between the upstream side and the downstream side of the tube (this is referred to as “pressure drop”) becomes large and the measurement of the injection rate becomes impossible.
Generally, the characteristic of the fuel injector is significant, because it is linked directly with the engine performance. Further, the fuel injector characteristic of measuring the injection rate which changes with time (instantaneous flow rate: fuel flow per unit time) is requested as the most important factor. However, with respect to a gaseous fuel injector, means for measuring the injection rate thereof has not been established as mentioned above. For this reason, steady flow rate data are obtained while keeping the injector with a valve opened and carrying out a steady-state injection and the injection rate on an occasion of an actual injection is estimated.
Also, a method can be taken into consideration in which a gaseous fuel injector is attached to a vacuum vessel and the pressure increase in the vessel is measured by carrying out injections, for example, 1000 times (“injection duration” of one stroke is fixed at, for example, 2 msec) into the vessel from the injector. Specifically, an average injection rate per stroke can be obtained by calculating pressure increase per stroke from the pressure increase caused by the injections 1000 times. However, it is not possible by this method to measure the instantaneous injection rate of the gaseous fuel. Until now, there has been no report that an instantaneous injection rate from a gaseous fuel injector has been measured.    [Non-patent Document 1] Hiroshi Hayashi “Bosch type injection rate meter” (Internal-combustion Engines Vol, 7, No. 12, pages 58 to 64)